Thermophotovoltaic generators for converting radiant energy from a thermal source into electricity have been known in the art. Conventional thermophotovoltaic generators generally involve a high temperature burner or radiator which becomes incandescent and illuminates thermophotovoltaic cells which convert a portion of the incident radiation into electricity. Numerous patents are directed to various burners and porous support media used in the combustion chamber. For example, U.S. Pat. Nos. 5,141,432, 5,160,254 and 5,080,577, all to Bell, et al., relate to an apparatus and method for conducting combustion within a porous matrix. This apparatus and method involves combusting a mixture of fuel and air in successive zones, the first zone where the mixture is fuel-lean, the second zone which receives the combustion products of the first zone and where the mixture is adjusted to be fuel-rich, and a third zone for receiving the combustion products of the second zone and where the mixture is adjusted to again be fuel-lean. Each of the zones is filled with a porous matrix having void spaces in which substantially all of the combustion occurs. The porous matrix comprises a foam made from zirconia or silica-alumina, or a packed bed comprising balls, saddles or rods.
U.S. Pat. No. 4,850,862 to Bjerklie, discloses a high efficiency, high temperature radiant heating system which has combustor/regenerator units each comprising a porous body with combustion supported in one layered zone and regeneration occurring in an adjacent layered zone in a subsequent cycle. This patent essentially discloses a radiant burner system which has paired or coupled sections. Conduits and associated controls cyclically direct combustible reactants to one combustor/regenerator section while removing combustion products from a companion or paired combustor/regenerator section. The roles of the combustor/regenerator sections are reversed by the controls and conduits on alternate cycles. The zones are preferably stacked, layered or disposed in plate-like fashion adjacent to each other. This geometry allegedly affords a compact, energy efficient construction. This patent discloses the advantages of employing regeneratively coupled porous body combustors which yield relatively low NO.sub.x because the useful temperature limits of the porous media are below that which would cause NO.sub.x generation. The porous media may be formed from a variety of materials such as silicon carbide frit which is held on a supporting form such as a screen, cloth, plate or other substance made from, for example, an alumina composite.
U.S. Pat. No. 4,836,862 to Pelka relates to a combustor/reactor for a thermophotovoltaic process which includes an insulated combustion chamber containing a combustion zone, first and second beds of refractory particles exposed to that zone, means for supplying a first combustion reactant to said zone, means for flowing a second combustion reactant to said zone via one of said beds during a first time and via the other of said beds during a second time, means for removing combustion products from said zone via the other of said beds during the first time and via the one bed during the second time, and thermophotovoltaic cell means exposed to heat radiated from said zone.
In general, NO.sub.x formation can be retarded by reducing the concentration of nitrogen and oxygen atoms at the peak of combustion temperature or by reducing the peak combustion temperature and residence time in the combustion zone. This can be accomplished by using combustion modification techniques such as changing the operating conditions, modifying the burner (emitter) design, or by modifying the combustion system. Of these techniques, modification of the burner design has been the most widely used. Low NO.sub.x burners are generally of the diffusion burning type, designed to reduce flame turbulence, delay the mixing of fuel and air and establish fuel-rich zones where combustion is initiated. Numerous patents relating to these techniques include U.S. Pat. No. 4,787,208 which discloses a low NO.sub.x combustor having a rich zone where NO.sub.x formation is inhibited by a low combustion temperature; U.S. Pat. Nos. 4,731,989, 4,535,165, 4,726,181, 4,730,599 and 4,285,193 all use catalytic stages in an effort to obtain low NO.sub.x combustion. U.S. Pat. No. 5,141,432 describes a low NO.sub.x burner apparatus having at least one combustion zone defined by a porous high temperature-resistant matrix and a cooling means mounted in proximity to an input end of the combustion zone.
Conventional TPV combustors or generators suffer from low efficiency, i.e. 10 to 15% efficiency, primarily due to an inability to recycle or recuperate the heat lost without overheating the system or subjecting the emitter materials to temperatures beyond their operating range. For example, U.S. Pat. No. 4,836,862 to Pelka describes thermal efficiencies which are relatively high, i.e. 72%, but this is achieved at temperatures exceeding 1900.degree. C. These temperatures are problematic not only from a materials limitation standpoint, but also from an emissions standpoint, whereby high NO.sub.x is created. The Pelka patent attempts to compensate for high NO.sub.x emissions through the use of specific catalysts. Due to the high temperatures, however, only specific ceramic oxides are useful. These materials must further be configured into specific geometric shapes, e.g. spheres, in order to tolerate thermal contraction and expansion forces which occur during the cycling of heat throughout the emitter bed. However, particle beds create a tortuous path through which the heated gases, i.e. combustion products, must move. Movement through such particle beds requires increased consumption of energy, since enough pressure must be exerted to physically push the gases through the bed. Thus, although these ceramic emitter geometries ameliorate thermal expansion and contraction forces, they create another problem in the form of pressure drops across the emitter and/or combustion chamber. This drop in pressure creates power consumption inefficiencies, whereby, for example, more fan power or energy is required to move the combustion products through the combustion chamber.
Conventional air/fuel ratios used in combustion systems generally range between 1:1 up to about 3:1. The air/fuel ratio is the ratio of the mass of air to fuel prior to combustion. Ratios greater than 3:1 are known to cause instability in the combustion flame which causes the flame to extinguish. Higher ratios of air to fuel, i.e., lean-burn ratios where significantly more oxygen and mass is present, would allow for maximum conversion of chemical energy to radiant energy. This in turn would result in a higher conversion of thermal energy to electricity, with the result being higher power densities and overall efficiency being greatly increased. Such lean-burn ratios have, heretofore, not been possible in conventional TPV systems. The present invention is also directed at providing for stabilized combustion of exceptionally high, lean-burn ratios at temperatures within the temperature resistance properties of current materials.
Combustion efficiency, however, is not the only problem to be solved in thermophotovoltaic processes. The creation of harmful by-products, i.e. NO.sub.x, is to be avoided and the challenge has been to create an apparatus and methodology which allows for efficient combustion, high density output with low NO.sub.x and carbon monoxide emissions. The present invention has disclosed a means of increasing the air/fuel ratio, which to a large degree governs the operating temperatures and hence the emission results. For low NO.sub.x , the mixture should be "lean" as opposed to "rich". These terms refer to ratios greater than 1.0 and less than 1.0, respectively. Whereas conventional metal combustors operate on rich or slightly lean ratios, more recent ceramic combustors allow for leaner mixtures and hence lower operating temperatures. As the operating temperature of the burner drops, less NO.sub.x emissions are produced. At leaner ratios, lower temperatures are possible, but the energy emitted also drops. Thus, it is apparent that there is a delicate and complex balance which must be achieved whereby high power density, i.e. high energy output, is obtained using a lean-burn combustion technique.
Heretofore, it has not been possible to maintain a stabilized combustion state using air/fuel ratios greater than about 3:1 without having a combustion temperature in excess of the temperature resistance capabilities of current ceramic materials. Additionally, to be an effective energy producing system, radiant and convective energy which is not harnessed should be recovered and returned to the system. Most preferably such recovered heat should be used to preheat incoming reactants, air and fuel. Preheating of reactants, however, is known to increase combustion temperature which beyond certain limits destroys the ceramics used in the system.
Thus, in one embodiment the present invention addresses this problem of burning reactant mixtures having air/fuel ratios of about 3:1 and greater at combustion temperatures which are useful for currently available ceramics, while utilizing heat recovered from the system and using the recovered heat to preheat the reactants without driving combustion temperatures above those useful for available ceramics.
The prior art has also failed to fully appreciate the need for various components of a TPV system to function in an integrated fashion. Consideration has not been given to maximizing energy output as a function of the total system, as well as minimizing energy losses in transferring energy between components. Instead, the art has focused on improving particular components, for example, a combustor or photocell, independent of their relationship to the efficiency and power density of the system as a whole. By building a system which addresses the problems associated not only with each individual TPV component, but also with their integration into a total system, the present invention represents a departure from the prior art. In addition to the unique, synergistic combination of system components and steps, improvements to various individual components represent other embodiments of the present invention.
It is clear that there is a need for such a TPV system and methodology. The present invention incorporates a unique combination of components to define a system, components and apparatus, as well as a method of achieving same.